Sickle cell hemoglobinopathies are hereditary disorders causing hemolytic anemia, painful vaso-occlusive crisis leading to severe organ failure, and a predisposition to infection. Hemoglobin F (Hgb F) reduces the sickle polymer formation in vitro, and patients with a high percentage of Hgb F tend to have milder disease. Hydroxyurea, an antimetabolite which inhibits DNA synthesis, has been shown to increase Hgb F in patients with sickle cell disease. Because no data is available regarding the pharmacokinetic behavior of hydroxyurea, or pharmacodynamic correlates with dose or concentration in children, we propose a pharmacokinetic study to elucidate this information.